1. Field of the Invention
This invention relates to a CATV converter adapted to separate a data signal necessary to descramble a scrambled television signal and a voice signal at a preferable S/N ratio with a simple configuration to produce both the signals.
2. Description of the Prior Art
A CATV broadcasting is provided for specific subscribers through a wire transmission different from an ordinary television broadcasting transmitted through a radio transmitter. The subscriber converts the CATV broadcasting transmitted from a broadcasting station to the frequency of a vacant channel of an ordinary television broadcasting channel by means of a CATV converter, and receives it by the ordinary television receiver.
The CATV broadcasting gives subscriber's codes and program contract codes to the individual subscribers so as to specify the subscribers, and transmits the codes together with the television signal from the transmitter to control and serve the individual subscribers.
There is a scrambling broadcasting of a charged program as one of the controls. This is a system that even the subscriber of the CATV who has not yet contracted cannot ordinarily receive the charged program different from the no charge service program.
There is as one of such scrambling broadcasting methods a transmission in which a television signal is scrambled to be transmitted by suppressing the synchronizing signal section of the television signal by the transmitter, and a data signal necessary to decode it is superposed on a voice carrier together with the subscriber's code and the program contract code to be transmitted. In this case, the CATV converter of the contracted subscriber can decode the synchronizing signal section according to the data signal to descramble the television signal, thereby receiving the normal video. However, the CATV converter of the uncontracted subscriber cannot descramble it, and cannot synchronize the received video in disorder, thereby disabling to normally receive the video.
FIG. 2 is a block circuit diagram showing a conventional CATV converter in which a data signal necessary to descramble a voice signal is separated by a split carrier system from a voice intermediate frequency signal to be decoded.
In FIG. 2, an intermediate frequency signal outputted from a tuner, not shown, is applied to a SAW filter 1, and only a voice intermediate frequency signal is extracted. The voice intermediate frequency signal is branched by a branch unit 2 to the signals for an AM modulation system and an FM modulation system, one of which is applied to an amplifier 4 of a data signal detecting IC 3, and the other of which is applied to a voice intermediate frequency amplifier 5. The voice intermediate frequency signal applied to the amplifier 4 is suitably amplified by an AGC control to be applied to a synchronous detector 6 and a carrier limiter 7. In the carrier limiter 7 is provided a tank circuit 8 for tuning with the voice intermediate carrier (e.g., 41.25 MHz) of the voice intermediate frequency signal to output the voice intermediate carrier to the synchronous detector 6. The synchronous detector 6 demodulates and outputs the data signal superposed with the voice intermediate signal by AM modulation by means of the synchronous detection according to the carrier, and applies an AGC signal to the amplifier 4. The amplifier 4, the synchronous detector 6 ad the carrier limiter 7 are contained in the data signal detecting IC 3, and attached with the tank circuit 8 thereto.
Further, the voice intermediate frequency signal applied to the voice intermediate frequency amplifier 5 is suitably amplified, further detected by the voice detector 9, and the voice signal superposed by the FM modulation is demodulated and outputted.
In the conventional CATV converter of such a configuration, the voice intermediate frequency signal is largely attenuated by the SAW filter 1 and the branch unit 2. Thus, the voice intermediate frequency amplifier 5 must be provided at the previous stage of the voice detector 9 of the FM modulation system, the AGC controller is also required in the voice intermediate frequency amplifier 5, the circuit configuration is complicated that much, and the converter has problems that it thus becomes expensive and cannot be reduced in size. Further, the converter also has problems that the S/N ratio of the data signal is wrong due to the detection of the data signal from the voice intermediate frequency signal attenuated by the SAW filter 1 and the branch unit 2. Moreover, the output of the SAW filter 1 is ordinarily outputted in balance, and the branch units 2 connected to the filter must input the signal in balance. However, when the branch units 2 input the signal in balanced, the configuration is complicated to be expensive. Then, the output of the SAW filter 1 is forcibly outputted in unbalance to use simple branch units 2. Thus, the converter has another problem that the common noise of the SAW filter 1 is large.